Process of dewatering sewage sludge



United States Patent Ofice 3,173,853 Patented Mar. 16, 1965 3 173 863PROCESS or nEWAinniNG SEWAGE SLUDGE Thomas H. Oster, 156 S. FranklinSt., Dearborn, Mich, and Jack E. (Iooper, 1375 W. Ann Arbor Trail,

droxide in a volume of 100 cubic centimeters of water was prepared.

An arbitrary test of sludge conditioning efficiency was used whichconsisted of treating a 100 cubic centimeter Plymouth Mich 5 sample ofsludge first was the ferrous chloride or ferric N0 Drawing Filed (kt 41963 sen 313,790 chloride or a mixture of these two salts, then adding7.5 2 Claims (CL 219 52 cubic centimeters of lime slurry followed by oneminute of gentle hand stirring. This entire mixture was then Thisinvention relates to the field of sewage disposal poured quickly into aneleven centimeter Buechner funand more particularly to a process for thepreparation of nel connected to an adequate vacuum system. The timessludges for dewatering by filtration to a water content noted extendfrom the pouring of the sludge into the sufiiciently low to permitincineration without the addifunnel until the vacuum in the systemdropped from the tion of extraneous fuel. maximum value of about 28inches of mercury to an This invention is applicable to activatedsludges or arbitrarily selected value of inches of mercury. At toprimary sludges either with or without the addition of 15 this stage asatisfactorily dry cake had been obtained. trickling filter sludges oractivated sludges. These sludges The results obtained in this series oftests may be sumhave been treated for at least thirty years by theaddition marized as follows:

Ferric FeOh, Ferrous FeOl Fe, total Test FeCla Iron Fe. percent FeCl:Iron Fe, percent; percent Time,

(ca) percent dry solid (cc.) percent dry solid dry solid seconds drysolid dry solid 0. 66 1. 92 None None None 0. 66 41 None None 1. 11 0.es 1. 49 o. as as o. 33 0. 96 0. 55 0. 33 0. 75 0. so as o. 23 0. so 0.50 0. 0. 66 0. 56 34 0. 1s 0. 44 0. so 0. 30 0. an 0. 59 37 0. 0s 0. 220. so 0. so 0. 66 0. so 40 0. 23 0. as 0. 0. so 0. 66 0. 53 3s 0. 23 0.cs 0. so 0. so 0. 66 0. 53 as 0. 33 0. 96 0. 30 0. 1s 0. 40 0. 51 3s 0.66 1. 92 None None None 0. 66 38 None None 1.10 0.66 1.49 0.66 l 35 of asolution of ferric chloride and lime followed by agitation. The actionof this combination of chemicals on the sludge is obscure, but it doespermit the resultant mixture of sludge, lime and ferric chloride to besatisfactorily dewatered, usually upon a rotary vacuum filter.

This invention is predicated upon our discovery that many sludgescontaining at least 3 percent of solids can be conditioned forfiltration more satisfactorily and economically if ferrous chloride issubstituted for part or all of the ferric chloride formerly used. Apressure differential of at least 3 inches of mercury should bemaintained across the filtering medium. Other than the substitution offerrous chloride for part or all of the ferric chloride the remainder ofthe process, that is, the addition of lime, the agitation and thefiltration remains the same.

A particular object of this invention is the conditioning of sludge sothat it may be dewatered by vacuum filtration to a degree that it willat least easily support its own combustion and preferably to a pointwhere it will be able to produce excess heat over and above thatnecessary to support its own combustion. This degree of dewatering ofsludge permits economies in the operation of the sewage disposalfacility because no fuel need be purchased to support the incinerationof the sludge.

This process is not to be confused with processes employing iron saltsto coagulate dilute sewage as taught in Muskat, 2,338,958, issuedJanuary 11, 1944, nor with the use of iron salts in the absence of limeto produce gas bubbles as shown in Halvorson, 2,590,964, granted April1, 1952.

As a demonstration of this invention we obtained from the Detroit SewageDisposal Plant a sample of primary sludge containing 0.68 pound ofsolids per gallon. A solution of ferric chloride was prepared bydissolving 10 grams of FeCl 6H O in water and making the volume up to100 cubic centimeters. A solution of ferrous chloride was prepared bydissolving 17.3 grams of FeCl -4H O in water and making the volume up to100 cubic centimeters. Similarly, a slurry of 10 grams of calcium hy- Ineach instance in the above tests where both ferric chloride and ferrouschloride were employed in the same test these chemicals were addedseparately except in Test No. 7 in which the chlorides were premixed.The 2.60 cubic centimeters of FeCl;, solution detailed above contains0.156 gram of FeCl on an anhydrous basis. Similarly, the 1.11 cubiccentimeters of FeCl solution contains 0.123 gram of FeCl on an anhydrousbasis.

It is conventional for sewage disposal plants to purchase ferricchloride in a forty percent water solution. Sewage plant operators mayeffect a substantial reduction in chemical costs in most cases bycontacting this ferric chloride solution with scrap iron or scrap steelfor a sufficient period of time to permit at least a substantial portionor all of the ferric chloride to be reduced to ferrous chloride with theconcomitant dissolution of the scrap iron. This reaction proceeds moresmoothly if the ferric chloride solution is strongly diluted with Waterprior to contacting it with the scrap iron or scrap steel. Cast iron orsteel borings or tin cans are adequate for this purpose. It is clearthat under some economic conditions it may be advantageous for theproducer of the ferric chloride to partially or completely reduce theferric chloride solution to ferrous chloride before shipping it to thesewage plant.

This application is a continuation-in-part of application Serial Number63,994, filed October 21, 1960, now abandoned.

We claim as our invention:

1. The process of dewatering a sewage sludge containing at least 3percent by weight solids comprising adding to the sludge lime and atreating agent containing ferrous chloride in an amount effective toobtain a dewatercd sludge of diminished water content as compared to theuse of equal weights of ferric chloride, whereby dimin ished amounts ofextraneous fuel will be required for incineration of the obtaineddewatered sewage sludge, agitating the mixture of sludge, lime andferrous chloride and finally dewatering the sludge by filtering themixture through a filtering medium while maintaining a pressuredifferential of at least 3 inches of mercury across. the filteringmedium.

2. The process recited in claim 1 in which the treating agent gansistsessentially qf ferrous chloride.

References Cited by the Examiner UNITED STATES PATENTS 2,338,958 1:/.4 4Muskat MOP-50 2,665,249 1/54 Zimmerman 21-OT53 Q HER E CE Bloodgopd:Water Works and Sewerage, 83, 251 (1936) s 4. Sewage Treatn1ent Works,Keefer, first edition, 1940, McGraW-Hill, New York, pp. 429-457, 478 185and 496-500 relied on.

Green: The Use of Steel Pickling Liquors for Sewage 5 udg Conditi n ng,Sewage Works o n 2 November 1949, pp. 1037-1049.".

Bargman ,et al.: Sludge Filtration etc., Sewage and Industrial Wastes,vol. 30, September 195:8, pp. 1079-1100.

Back et all: Sew. and Ind. Wastes, 1955, vol. 27,, pp.

MORRIS O; WOLK, Primary Examiner.

1. THE PROCESS OF DEWATERING A SEWAGE SLUDGE CONTAINING AT LEAST 3PERCENT BY WEIGHT SOLIDS COMPRISING ADDING TO THE SLUDGE LIME AND ATREATING AGENT CONTAINING FERROUS CHLORIDE IN AN AMOUNT EFFECTIVE TOOBTAIN A DEWATERED SLUDGE OF DIMINISHED WATER CONTENT AS COMPARED TO THEUSE OF EQUAL WEIGHTS OF FERRIC CHLORIDE, WHEREBY DIMINISHED AMOUNTS OFEXTRANEOUS FUEL WILL BE REQUIRED FOR INCINERATION OF THE OBTAINEDDEWATERED SEWAGE SLUDGE, AGITATING THE MIXTURE OF SLUDGE, LIME ANDFERROUS CHLORIDE AND FINALLY DEWATERING THE SLUDGE BY FILTERING THEMIXTURE THROUGH A FILTERING MEDIUM WHILE MAINTAINING A PRESSUREDIFFERENTIAL OF AT LEAST 3 INCHES OF MERCURY ACROSS THE FILTERING MEDIUM.